Photorespiration and Carbon Limitation Determine Productivity in Temperate Seagrasses

The gross primary productivity of two seagrasses, Zostera marina and Ruppia maritima, and one green macroalga, Ulva intestinalis, was assessed in laboratory and field experiments to determine whether the photorespiratory pathway operates at a substantial level in these macrophytes and to what extent it is enhanced by naturally occurring shifts in dissolved inorganic carbon (DIC) and O-2 in dense vegetation. To achieve these conditions in laboratory experiments, seawater was incubated with U. intestinalis in light to obtain a range of higher pH and O-2 levels and lower DIC levels. Gross photosynthetic O-2 evolution was then measured in this pretreated seawater (pH, 7.8-9.8; high to low DIC:O-2 ratio) at both natural and low O-2 concentrations (adjusted by N-2 bubbling). The presence of photorespiration was indicated by a lower gross O-2 evolution rate under natural O-2 conditions than when O-2 was reduced. In all three macrophytes, gross photosynthetic rates were negatively affected by higher pH and lower DC. However, while both seagrasses exhibited significant photorespiratory activity at increasing pH values, the macroalga U. intestinalis exhibited no such activity. Rates of seagrass photosynthesis were then assessed in seawater collected from the natural habitats (i.e., shallow bays characterized by high macrophyte cover and by low DC and high pH during daytime) and compared with open baymouth water conditions (where seawater DIC is in equilibrium with air, normal DIC, and pH). The gross photosynthetic rates of both seagrasses were significantly higher when incubated in the baymouth water, indicating that these grasses can be significantly carbon limited in shallow bays. Photorespiration was also detected in both seagrasses under shallow bay water conditions. Our findings indicate that natural carbon limitations caused by high community photosynthesis can enhance photorespiration and cause a significant decline in seagrass primary production in shallow waters.